Add support for Aria2 RPC

[hguandl]

There are several reports about the download speed would slow down gradually, especially in Bilibili:
#275 #143 #128

I am not an expert in Go and have no idea to improve the application itself. So I tried to use third-party applications to download.

aria2 is a very famous and lightweight command-line download utility. It has an interface of RPC, which means we can merely send a request to make it work.

  -aria2
    	Use Aria2 RPC to download
  -aria2addr string
    	Aria2 Address (default "localhost:6800")
  -aria2method string
    	Aria2 Method (default "http")
  -aria2token string
    	Aria2 RPC Token

Here is the demo:

• Use built-in download:
  

• Use aria2c (I use a WebUI, AriaNg to watch the speed):
  

I know that this breaks the independence of annie, but it is an effective workaround to improve the download speed. In addition, I only implemented the download request, leaving the post process empty. Perhaps I would make the function more completed some day or just make it a plugin for annie.

Last but not least, thank you for developing this convenient tool.

[codecov]

Codecov Report

Merging #284 into master will increase coverage by 1.19%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #284      +/-   ##
==========================================
+ Coverage   72.07%   73.26%   +1.19%     
==========================================
  Files          26       26              
  Lines        1898     1919      +21     
==========================================
+ Hits         1368     1406      +38     
+ Misses        381      358      -23     
- Partials      149      155       +6

Impacted Files	Coverage Δ
downloader/downloader.go	44.89% <0%> (-4.21%)	⬇️
extractors/twitter/twitter.go	68.49% <0%> (+52.05%)	⬆️

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[iawia002]

Thanks for this! I will do some documentation and format related work tomorrow night, and then I will draft a new version.

[M1178475702]

I have saw the code about download.
I find that when download paylist video, there are downloaed one by one, not concurrent.
I wonder why not change it to conconcurrent?
If i modify it to concurrent, something (may be bug) will occure?